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Quoting Gary:
"Coriolis is a fictitious force use to explain the perceived curve of the 
flight path as observed by an observer on earth of other rotating frame 
of reference."

Coriolis acceleration is a "frame-dependent" acceleration. That is, it depends 
on the choice of coordinates that you use to define motion. The expression 
"fictitious force" is a technical term intended to describe this 
frame-dependence. Just so there's no misunderstanding, it DOES NOT mean that 
the Coriolis acceleration is "fictional" in the ordinary sense of the English 
word. Of course, Coriolis acceleration is "real" in the sense that in many 
circumstances you find that the natural coordinates to describe the problem 
are rotating coordinates. The weather on Earth or any other planet, for that 
matter, is nearly impossible to understand in non-rotating coordinates but 
quite easy to understand in coordinates which rotate.

From a modern perspective (modern = since the early part of the 20th century, 
after the development of general relativity), even the common acceleration of 
gravity near the surface of the Earth (at a single point) at 9.8 m/s^2 or 32 
ft/sec^2 is a "fictitious force" since it can be eliminated by going to a 
frame of reference which is accelerating toward the Earth's center, in other 
words a freefall frame. And sure enough, if you place an aircraft on a 
parabolic trajectory with that acceleration, gravity disappears and 
passengers are rendered completely weightless exactly as if they are in orbit 
(until the plane's acceleration trajectory is changed). It is not "as if" 
there is no gravity; in that frame of reference, there really is "no gravity" 
apart from local tidal accelerations.

Many people are able to explain the origin of the Coriolis acceleration by 
describing how it appears in an inertial, non-rotating frame of reference. In 
such a frame, the object moves on a straight line "while the Earth turns 
beneath it". This is very important information, of course, but it is a 
derivation, like an "etymology". It tells us why this acceleration must exist 
in non-inertial frames of reference, but it doesn't mean that it is fake or 
"fictional". And you surely wouldn't want to revert to the derivation once 
you understand why it works. 

In many ways, the expression "fictitious force" in physics has caused as many 
problems as the expression "imaginary number" in mathematics. Neither of 
these concepts are invalid or fictional or "un-real". For modern mathematics, 
so-called imaginary numbers have no defects, no lack of reality to them. They 
are what they are: solutions to algebraic equations. Likewise in physics, 
so-called fictitious forces are not forces to be avoided or treated as 
"un-real". They are what they are: forces arising from the choice of 
coordinates.

-FER





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